1. INTRODUCTION Voltage sags are short duration reductions in rms voltage, mainly caused by short circuits and starting or large motors. The large interest in voltage sags is due to the problems they cause on several types of equipment. Specially adjustable-speed drives, process-control equipment and computers are notorious for their sensitivity. Some pieces of equipment trip when the rms voltage drops below 90% for longer than one or two cycles. Such a piece of equipment will trip, tens of times a year. If this is the process-control equipment of a paper mill, one can imagine that the damage due to voltage sags can be enormous. Of course a voltage sag is not as damaging to industry as a (long or short) interruption. But as there are far more voltage sags than interruptions the total damage due to sags is still larger. Another important aspect of voltage sags is that they are rather hard to mitigate. Short interruptions and many long interruptions can be prevented via rather simple, although relatively expensive, measures in the local distribution network. However voltage sags at equipment terminals can be due to short circuit faults hundreds of kilometres away in the transmission system. It will be clear that there is no simple method to prevent them. An example of a voltage sag in shown in Fig. 1 . We see that the voltage amplitude drops to a value of about 20% of the pre-event voltage for about two cycles. After these two cycles the voltage comes back to about the presag voltage. This magnitude and duration are the main characteristics of a voltage sag. Both will be discussed in more detail in the 'forthcoming sections. We can also conclude from Fig. 1 that magnitude and duration do not completely characterise the sag. The during-sag voltage contains a rather large amount of higher frequency components. Also the voltage shows a small overshoot immediately after the sag. In how far these higher frequency components are of any influenceon the equipment behaviour due to sags, remains a point of discussion.